Bioavailability: Why the Carnivore Argument Gets It Wrong

By: Marcy Schoenborn

If you spend any time in nutrition debates right now, you will hear the word bioavailability used constantly.

Often it is presented like this:

“Animal foods are superior because their nutrients are more bioavailable. Plant foods aren’t.”

At first glance that sounds scientific. But in reality, the argument is often oversimplified and frequently misused.

To understand why, we need to look at what bioavailability actually means — and more importantly, what it does not mean.

What Bioavailability Actually Means

Bioavailability refers to the proportion of a nutrient that the body absorbs and uses after digestion.

It is not simply how much of a nutrient is present in a food.
It is how much ultimately reaches circulation and becomes available for biological processes.

Several factors influence bioavailability:

• digestion
• enzyme activity
• gut microbiome
• nutrient interactions
• cooking methods
• overall metabolic health

In other words, bioavailability is context dependent, not a fixed number attached to a food.

How the Term Gets Misused

In many carnivore or heavily meat-focused discussions, bioavailability is used as a scorecard to rank foods.

The argument usually goes something like this:

• animal nutrients are more bioavailable

• plants contain “anti-nutrients”

• therefore plants are inferior foods

But biology doesn’t work that simply.

A food’s value cannot be reduced to how quickly a nutrient is absorbed.

To see why, we need to examine something surprising.

The Bioavailability Paradox

Some of the most metabolically beneficial compounds humans consume have almost zero direct bioavailability.

Two of the biggest examples are:

• dietary fiber
• resistant starch

From a traditional absorption perspective, these compounds appear useless.

Yet they play a major role in metabolic health.

Fiber: A Nutrient We Don’t Actually Absorb

Humans do not possess the digestive enzymes required to break down fiber.

When fiber passes through the small intestine, it remains largely intact.

That means its direct bioavailability is essentially zero.

If we applied the same logic often used in carnivore arguments, we would conclude:

Fiber is useless because it isn’t absorbed.

But that conclusion would be completely wrong.

What Happens to Fiber Instead

When fiber reaches the colon, it becomes fuel for the gut microbiome.

Beneficial bacteria ferment fiber and produce compounds called short-chain fatty acids (SCFAs).

The most important ones include:

• butyrate
• propionate
• acetate

These compounds are absorbed and used by the body.

And they have powerful effects on metabolism.

Why Butyrate Is So Important

Butyrate is the primary fuel for the cells lining the colon.

It helps:

• maintain gut barrier integrity
• regulate immune responses
• reduce inflammation
• support mitochondrial function
• influence insulin sensitivity

Butyrate also influences gene expression by affecting epigenetic signaling pathways.

In other words, a compound created from fermented fiber can actually influence how certain genes are turned on or off.

All of this begins with something humans cannot digest themselves.

Resistant Starch: Another Example

Resistant starch behaves in a similar way.

It is a form of starch that escapes digestion in the small intestine and reaches the colon intact.

Foods that contain resistant starch include:

• legumes
• oats
• cooked and cooled potatoes
• green bananas
• some whole grains

Like fiber, resistant starch becomes food for gut microbes.

Research shows resistant starch fermentation can help:

• improve insulin sensitivity
• increase satiety hormones
• support GLP-1 signaling
• improve gut microbial diversity

Again, none of this depends on direct nutrient absorption.

Nutrition Is More Than Absorption

The bioavailability argument assumes that nutrition works like a simple input-output system:

Food → absorption → health.

But the human body operates through multiple interacting systems, including:

• nutrient absorption
• microbial fermentation
• hormonal signaling
• immune regulation
• mitochondrial metabolism

Many plant foods influence several of these systems at once.

When we reduce nutrition to simple absorption percentages, we miss a large portion of how food actually affects biology.

The Role of the Gut Microbiome

Another major piece of this puzzle is the microbiome.

The bacteria living in the human digestive tract play a huge role in metabolizing plant compounds.

Many plant phytochemicals become biologically active only after microbial processing.

For example, gut bacteria transform certain plant compounds into metabolites that influence:

• inflammation
• blood sugar regulation
• mitochondrial activity
• immune function

This means some nutrients are not immediately bioavailable but become bioactive later through microbial metabolism.

The Importance of Dietary Diversity

Research into the microbiome has repeatedly shown that dietary diversity supports microbial diversity.

Higher microbial diversity is associated with:

• improved metabolic health
• lower inflammation
• stronger immune regulation
• reduced risk of obesity and metabolic disease

And the strongest predictor of microbial diversity?

Fiber diversity from a wide range of plant foods.

Restrictive diets that eliminate plant foods often remove many of these microbial substrates.

The Bigger Picture

Bioavailability is an important concept in nutrition science.

But it is only one piece of a much larger biological system.

A nutrient being absorbed quickly does not automatically make a food healthier.

Likewise, a compound that is not directly absorbed may still exert powerful effects through the microbiome or metabolic signaling pathways.

Nutrition is not just about what enters the bloodstream immediately.

It is about how food interacts with the entire biological network that regulates metabolism, inflammation, energy production, and long-term health.

A Simple Way to Think About It

Some of the most powerful compounds in nutrition aren’t absorbed directly.
They work by feeding beneficial microbes, influencing metabolic signals, and supporting the body’s regulatory systems.

When nutrition is viewed through this broader biological lens, the idea that plant foods are inferior because of “low bioavailability” simply does not hold up.

References

Canfora, E. E., et al. (2015). Gut microbial metabolites in obesity and type 2 diabetes. Nature Reviews Endocrinology.

Koh, A., et al. (2016). From dietary fiber to host physiology: short-chain fatty acids as key bacterial metabolites. Cell.

Makki, K., et al. (2018). The impact of dietary fiber on gut microbiota in host health. Cell Host & Microbe.

Slavin, J. (2013). Dietary fiber and body weight. Nutrition.

Deehan, E. C., & Walter, J. (2016). The fiber gap and the disappearing gut microbiome. Cell Host & Microbe.